The invention relates to increasing the dynamic range of a detector. In particular, increasing the dynamic range of a detector used in a mass spectrometer system.
The linear dynamic range of mass spectrometers can often be limited by the ion detection system. Ion sources are now intense enough that the number of ions delivered to the detector is large enough to saturate the detection system. This issue, in some respects, is more critical in ion trap instruments, which attempt to regulate the exact number of ions contained in the trap using a prescan measurement technique. In this case, any saturation effect of the detector would result in substantial space charge effects in the desired mass spectrum. Consider for example, the analytical scan for which a prescan experiment is performed prior to the analytical scan in order to obtain a measurement of the flux of the ion beam. The measurement can then be used to determine the ion accumulation time used for the analytical scan. However, by using a fixed prescan ion accumulation time, there is a possibility that one or more of the peaks in the prescan will saturate the detector electronics if the ion current from the source is high. Under these conditions, the measured total ion current (TIC) will be less than the actual TIC. Use of this low TIC results in the calculation of an ion accumulation time for the subsequent analytical scan which is erroneously high, causing possible space charge to occur and therefore an overall reduction in performance in the mass spectrometer.
In the case of a typical quadrupole ion trap mass spectrometer, as the API source has become more efficient, the normal prescan ion accumulation time of 10 ms can cause the electrometer to be saturated by the current produced by the electron multiplier. The saturation is even more likely to occur during the prescan measurement primarily because of the higher scan rate (0.015 ms/amu which is 12 times the analytical scan rate) ejects ions faster, resulting in narrower, taller peaks.
Again, in the case of an ion trap mass spectrometer, the result is that the ion trap can be overfilled for the subsequent analytical scan, resulting in reduced performance.
For linear ion traps, the saturation problem is more severe for several reasons. First, a linear trap fundamentally can hold more ions (has a higher dynamic range) and therefore will deliver more ions to the detector. Second, the linear ion trap can be operated with two detectors, which then doubles the detected current. Third, the higher resolution of the current linear ion traps allows for even higher scan rates during the prescan (20-50 times the analytical scan rate) and higher scan rates produce higher detected currents (narrower but taller).
In some instances, the dynamic range limitation of the detection system can be caused by the saturation of the analog to digital conversion component (ADC). For example, a 16-bit analog to digital conversion (ADC) is limited to a maximum of 4.8 orders of magnitude (log 216). This is because a 16-bit ADC has a range of possible digital output values from 0 to 65535 counts. When using such a component, one must typically adjust the gain of the detector, or that of the amplifier between the detector and the ADC input so that a single ion pulse amplitude produces a signal at the ADC input that corresponds to several digital counts. This is so that most of the single ion pulse amplitudes are large enough to register at least one bit on the digital counter. Otherwise, the single ions that produce output pulses with amplitudes that fall below that threshold will not be recorded, resulting in an error in the intensities measured. So in practice, a 16-bit ADC has less than 4.8 orders of magnitude of dynamic range. Typically, the effective dynamic range would be about 3.5 orders of magnitude.
When the ADC at the output of the ion detector has insufficient dynamic range, several methods can be used to improve it.
First, existing methods of increasing this range have included multi-anode electron multipliers. Here, different percentages of the ion signal are collected on different anodes, and one anode collects a larger percentage of the ion signal than the other. Multiple electrometers are used to measure these currents. The electrometer with the best measurement is then used. It can be difficult to keep the relative gain between these channels constant though, and the systems are more complex because they require two, or more, ADCs.
Second, non-linear amplifiers can be used. With these, the gain changes as a function of the input signal. For example, if the output of the amplifier is the inputA where 0<A<1, then the input signal range will be compressed into a narrower output signal range. This allows a wider input signal range to fit within the dynamic range of the ADC. However, resolution is reduced. This makes the quantization error worse across the entire input signal range compared to linear amplifiers where A=1. On the other hand, logarithmic amplifiers can be used where the output is B*log(input)+C where B and C are constants. With proper choice of B and C, the quantization error at low input signals is actually improved compared to linear amplifiers. However, the quantization error will be worse at high input signals compared to linear amplifiers. Unfortunately, logarithmic amplifiers often have low bandwidth, which adversely affects dynamic range. They also have poor temperature stability making them complicated and expensive to produce.
Third, ion detection systems have been used that switch the gain of the signal based on the input signal. For example, the gain of the analog amplifier can be adjusted. These systems typically have two or more gain stages that can be selected from. The problem is that the input signals can change rapidly and typically the switching circuit is not fast enough to keep up. In addition, such systems are typically expensive and complicated to produce.
There is a need to develop detection systems that are able to operate over a high dynamic range, able to detect particles over a wide range of intensities, from weak to strong intensities without suffering from saturation or an overly low detection threshold in the noise band. Furthermore, there is a need for a detection system that is capable of operating in real-time, enabling high speed detection to be facilitated whilst once again, operating under conditions such that saturation or low detection threshold levels are not an issue. Methods and apparatus' providing a simpler method of increasing the dynamic range while maintaining good resolution are required.